Important properties of a substance or medium are the viscosity, the shearing stress, the specific weight and whether the substance shows Newtonian behavior. These properties are measured by means of sensors or devices. This device moves a mass or object in or on the medium in order to measure these properties on the basis of the movement actually followed as a response to an excitation. This movement can be followed in all directions, i.e. in six freedoms of movement or combinations thereof. Most sensors operate with a pivot arm which forms an oscillating object and is in contact with the medium. In the known device the measurement results and conclusions to be derived therefrom about the properties of the medium are however not very reliable.
In addition, each oscillator has a resilient portion. This resilient portion is always susceptible to temperature fluctuations. The material of the spring expands or contracts under the influence of changing temperature. The dimensions of the spring hereby change and the frequency thereof will change. The Young modulus of the material, and thereby the spring constant, will also change due to change in temperature.
The pivot arm which is in contact with the medium will also change in physical size at different temperatures, whereby measurements, which correspond to and are dependent on the contact surface with the medium, are therefore also temperature-dependent.
The measurement of a shearing stress of a medium can take place by measuring the friction which the medium causes when in contact with the pivot arm in the form of an oscillating portion.
Many methods are possible for this purpose, for instance determining the damping. This is possible by measuring an amount of energy necessary to keep the oscillating system in oscillation when the oscillating system is driven near its resonance frequency. It is of course also possible to keep the added amount of kinetic energy constant and to measure changes in the movement of the pivot arm.
It is also possible to provide a fixed amount of kinetic energy at the resonance frequency of the oscillating system, wherein a change in the movement of the pivot arm caused by the medium can be measured. It is of course also possible to vary the amount of added kinetic energy so that there is a constant movement. All these methods have been known since 1950 using the Bendix type viscosity meter.
When an oscillating mass in the medium displaces the medium as a result of its own movement, the measurement will always be a combined measurement of shearing stress and specific weight.
In order to measure the specific weight of the medium a displacement of the medium must take place, which can be caused with the pivot arm as oscillating portion in the medium. This measurement can take place in the same way as determining of the shearing stress. When the pivot arm also displaces medium due to its movement in the medium, depending on the size or dimension of the frontal surface area in the direction of movement through the medium, a force will be created in the opposite direction.